限制对象的特征？

Question

有没有办法限制特征,使其只能混合到对象中？例如

trait OnlyForObjects {
  this: ... =>
}

object Foo extends OnlyForObjects  // --> OK

class Bar extends OnlyForObjects   // --> compile error

Answer 1

是! 有晦涩难懂的,大部分都没有记载scala.Singleton:

scala> trait OnlyForObjects { this: Singleton => }
defined trait OnlyForObjects

scala> object Foo extends OnlyForObjects
defined module Foo

scala> class Bar extends OnlyForObjects
<console>:15: error: illegal inheritance;
 self-type Bar does not conform to OnlyForObjects's selftype OnlyForObjects
 with Singleton
       class Bar extends OnlyForObjects
                         ^

它在语言规范中提到了几次,但是甚至没有出现在API文档中.